Sometimes after finding all of the hidden and naked singles, you will reach the point where you will still have cells with multiple candidates. At this step you will need to fill such cells with pencil marks.
Here come «naked pairs» and «naked triples» - basic solving techniques which will help you to eliminate some candidates and solve medium sudoku puzzles @ thesudoku.app.
A «naked pair» is two cells, both in the same row, column or 3x3 unit, which have same two candidates left. It is clear that these candidates can be eliminated from any other cell in this row, column or 3x3 unit respectively.
A «naked triple» is three cells from the same row, column or 3x3 unit, which have same three candidates left in different combinations. Since none of these candidates can be the solution in any other cell in this row, column or 3x3 unit, you can remove pencil marks from the other cells.